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Folds
Chapter 10
Folds Chapter Outline Excerpts Simplified from Published Geological Survey Maps Bristol District SW, UK East Falkland Islands, South Atlantic
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When a geology student confronts a geological map for the first time, the geological boundaries may appear as a bewildering pattern. Soon we learn to recognize that the contortions and bends of boundaries are due the topographic surgery of the solid geology, especially if the map is at a small scale. Even a horizontally bedded sequence shows a complex outcrop pattern; it follows the contortions of the topographic contours precisely. Why is it that we are so comfortable with topographic maps and topographic contours yet initially perplexed when confronted with a geological map? Like the pioneer, William Smith, we learned to interpret the interference of geological layers with the topography in order to understand their relative ages. Perhaps by now we realize Smith’s genius. To paraphrase Charles Darwin, we do not understand simply by looking or sketching; we understand truly when observations are perceived by the “inner eye of reason”. We often take important simple ideas for granted; but remember that before someone introduced them there was usually ignorance or misunderstanding. So far, we discussed strata whose map patterns are sinuous due to topography. We understood in those examples that the strata were essentially planar and not actually contorted. Now we meet cases where the strata are contorted; they exhibit folds, undulations of the layers due to tectonic movements that usually commence with the process called buckling by structural geologist. Other processes, usually shortening, often subsequently amplify the buckles. Bucking has the attractive property that the undulations of layers have an approximately regular waveform as long as the layers have constant thickness and uniform lithology. Thus, geologist will talk of the wavelength and amplitude of folds, as though they were sinusoidal waveforms in physics. Of course, such precision is lacking in nature Understanding Geology Through Maps. http://dx.doi.org/10.1016/B978-0-12-800866-9.00010-7 Copyright © 2014 Elsevier Inc. All rights reserved.
North West Cyprus Pierre Greys Lakes, Alberta Pembroke East, S. Wales Gananoque Area, Ontario
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and some folding processes produce highly irregular nonperiodic undulations of layers, even if they are of uniform thickness and lithology (e.g., disharmonic folds, ptygmatic folds, etc. in high-grade rocks). Since any folds produce different changes in orientation in different parts of the bedrock they are considered as examples of heterogeneous strain. Homogeneous strain occurs where straight lines remain straight and parallel lines remain parallel. Homogeneous strain is rather rare and usually only approximated at some certain scale. Every rock is heterogeneously strained at some scale or other. Folds may occur in soft sediment due to slumping. More commonly, we recognize them as the result of tectonic movements in sedimentary rock, in metamorphic rock of every grade, and in some layered igneous rocks. Folding is a common response of rocks and sediments to motions slower than those involved with faulting and usually with stresses that are much lower than those causing faulting. Generally, they result from steady-state metamorphic flow that also produces schistosity or cleavage approximately parallel to the axial planes. A fabric lineation may also be produced parallel to the maximum extension direction that is normally at a high angle to fold axes. A fold is a wrinkle whose alignment or axis is perpendicular, or nearly so, to the shortening direction. Axes are not necessarily perpendicular to the maximum compressive stress since folds evolve slowly; in contrast, a state of stress is an instantaneous phenomenon. Folds accumulate increments of strain over a long period to produce a state of heterogeneous finite strain. Thus, as explained in Chapter 9, finite strain structures such as folds cannot be related to stress axes, which are instantaneous changing phenomenon (Figure 9.6). 141
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Calculations suggest that cliff-sized folds along the Devonshire coast of England took hundreds of thousands of years to form. On the other hand, faults may propagate at the velocity of sound in rock (several kilometers per second) and movements on them may be measurable in meters per second. Huge flat-lying overfolds (recumbent folds or nappe folds) form in metamorphic rocks with stress differences between minimum and maximum compression of less than 100 bars (10 MPa) within a few million years. In contrast, brittle failure causing fault motion may require more than 1 kbar (>100 MPa) differential stress and occur in seconds. The shapes that folds may take and the orientations of those folds give rise to a wide range of geometric possibilities that we may meet. In this chapter, we shall deal with folds at a rather simple level although the description of folds in the diagrams is somewhat more encompassing than we need and is given for your future reference. Even then, the simplification is made that the folds are regular geometrical forms; in particular, that they preserve the same degree of folding along their hinge-line length. Initially, we will consider only angular (chevron) folds since these have planar limbs (Figure 10.1(a)). Folds with curving hinges and curved limbs are considered later but a simple classification based on interlimb angle and axial plane dip is given in Figure 10.1(b and c). Initially also, we consider only folds that have a horizontal plunge, i.e., the fold axis is not tilted. Initially also, we restrict ourselves to upright folds, ones in which the axial plane is vertical. Thus, after reviewing the diagrams, we will first tackle the simplest fold map construction where the folds are 1. a ngular (=flat limbs, sharp hinge), 2. upright (=axial plane of symmetry vertical), and 3. nonplunging (=axis or hinge line is horizontal). These diagrams will be reviewed but here we address the question that may have arisen earlier in your minds. Planar, nonfolded strata crop out along contorted patterns, dictated by topography. How then will we determine from map view, if the sinuous outcrop patterns are due to folding and topography or solely due to topographic effects? This problem is not as great as it may seem. The following steps are useful. 1. C an a bend of strata be recognized anywhere? a. Consider the case where the topography is negligible, as in most regional maps such as scales of 1:100,000, 1:200,000, etc. In these cases, any substantial bends in a stratum will indicate the hinges of plunging folds. Where fold hinges are horizontal or plunge very gently, map view may not reveal the closure or curvature of the fold hinge. However, in that case, sequences of strata (e.g., A–B–C) will repeat in mirror image order (i.e., C–B–A) across the map.
Understanding Geology Through Maps
b. Where topography is a consideration, e.g., at scales of 1:50,000 or 1:10,000, reading the sense with which beds “climb” or “descend” slopes will reveal the presence of a fold. For example, even a fold with a horizontal axis will expose the closure (bend) of the hinge region where the fold axis meets a topographic slope. Inspect the trend of the strata; this must correspond broadly to their strike and broadly to the fold axis trend. Where the fold meets a slope at a high angle, the same bed may change elevation as it crosses the slope to form a downslope bend (synform) or an upslope bend (antiform). This is a robust mapreading approach that simply detects the presence of a fold by inspection. It rarely reveals fundamental information concerning the shape or orientation of the fold since the map view contortions of a folded bed are dependent of the topography. However, we shall see that simple constructions reveal quantitative information on the fold’s shape and orientation. Eventually, the geologist learns to perform similar qualitative interpretations mentally. 2. Is the stratigraphic sequence repeated in two nearly opposite directions? A fold necessarily bends the strata so that the two flanks (limbs) tilt the strata in nearly opposite directions. When exposed by erosion, the sequence of the beds crops out in mirror image on either side of the fold’s axis. For example, the beds get younger away from the core of an anticline; the beds get older away from the core of a syncline. Consider a sequence of beds, A is older than B which is older than C. Now, inspect the map; does the sequence A → B → C occur in one direction (say, younging eastwards) in one part of the map and does it then occur younging eastwards C ← B ← A, in another part of the map? This would indicate that the strata dipped in opposite directions due to folding. 3. Are dip-and-strike symbols present? On some maps, dip-and-strike symbols will provide the obvious clue to the presence of folds. An antiform separates obviously opposed dip directions and face-to-face dip directions occur on either flank of a synform. A further clue to the presence of folds may be provided by antiparallel, or nearly opposed, younging arrows, Y [younger beds at the foot of the letter symbol], mainly on maps of very complexly deformed rock. However, some caution may be required in the use of structural symbols, depending on the scale of the map and the size of the folds. For maps that cover very large areas (1:50,000 and 1:100,000), the symbol must be very carefully placed to avoid misleading the map reader since actual outcrops are smaller than the symbol. It should also be remembered that the symbol may cover a huge ground area, of several hundred square meters. A common convention is that the tip of the symbol locates the site of the observation. This does not help very much in many cases; I have
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FIGURE 10.1 Classification of simple fold forms. Most natural folds have much more complex curving shapes (a) (b) and (C) identified in drawing.
seen regional maps (1:100,000 and larger) on which the area covered by the symbol is so large that it obscures the area in which the fold crops out. This is generally the reason why regional maps omit all small-scale structure symbols of any kind. Such large-scale maps also present a further difficulty; topographic relief is usually so
small in comparison to the map scale that we cannot use topography–geology relations to deduce reliable geological relations such as dips and sequence. Such largescale maps tend to become diagrams that only show geographical distributions of rocks. Three-dimensional interpretation requires some further skills.
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4. C onstruct stratum contours! As with faults, the only reliable way to determine the presence, nature, and effects of folds is to construct stratum contours (Figure 10.2). This technique works where we have small scales and topographic contours. Structure contours will trace out the form of a bed as if it cropped out on a hypothetical horizontal plane (or mine plan). Clearly, the same stratum contour (e.g., sandstone–limestone, 350 m) will appear twice for each fold, once on each fold limb. With good “book keeping”, by labeling each stratum contour carefully and working in erasable pencil, we may interpret such maps uniquely. A common elementary confusion is that when the intersections of topography and a bedding plane are determined, upon first sight there may appear to be ambiguity in the way they are connected. The correct of possibilities will be parallel to the respective flanks of the fold. The alternative set of misconstructed stratum contours will cut across the symmetry of the fold and be inconsistent with topography. Recall earlier that we learned that a geological boundary appears only at the topographic surface if is justified by the intersection of a topographic contour and stratum contour of the same elevation. 5. Where folds plunge (Figure 10.3), the stratum contours from opposing flanks will meet at the hinge
Understanding Geology Through Maps
(Figure 10.3(b and c)). The separation of the stratum contours along the axial trace permits the plunge of the fold to be determined. This is equivalent to determining the dip of the hinge of the fold along the axial trace (Figure 10.4). Some natural structures flag the geometry of the major fold. For example, cleavage or schistosity is commonly parallel to the axial planes of fold (Figure 10.4(a)). In a similar fashion, minor folds on the flanks of major folds will define the plunge of the major fold (Figure 10.4(b)). Note that the asymmetry of the minor folds changes from one major flank to the other; commonly, these are termed “S” and “Z” asymmetry folds. However, their exact appearance on the map also depends on the slope of the topographic surface on which they appear; this will become apparent in later exercises (e.g., Figure 10.14). Natural folds are not as simple or “perfect” as the examples in the preceding diagrams. An example of a natural fold is mapped in Figure 10.5. Note that the main synform is accompanied by two minor folds on its southern flank. However, these folds do not persist for a long distance along their axial traces. Such folds are termed “disharmonic” as minor axial traces eventually disappear, merge, or are faulted out.
FIGURE 10.2 Isolation of folds (in this case an antiform) using structural contours. (a) Simple antiform (b) Same with stratum contours (c) Map view of structure contours.
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FIGURE 10.3 (a) A simple plunging angular antiform (b) structure contours (c) map view.
FIGURE 10.4 A rounded antiform; note the curving structural contours. (a) three-dimensional structure (b) map.
Some simple exercises with folds illustrate these concepts. First, in Figure 10.6, we meet upright folds without plunge; structure contours will be parallel and straight. Determine the stratigraphic order by examining
topographic slopes (shale and sandstone are illustrated). Determine the dip angles at the locations marked with dipand-strike symbols. Using labeled stratum contours, determine the presence of any folds. Mark the axial traces of
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FIGURE 10.5 An example of a natural fold, from northern England. Note the variation of limb thicknesses and the fact that the minor folds are disharmonic, i.e., they vary in amplitude along their axial planes, causing them to disappear or merge.
the folds following the examples given. Draw a true-scale cross-section LR with L on the left, extrapolate the geology above and below the topography. The cross-section is perpendicular to the fold axes and thus it will show a true plunge profile.
Figure 10.7 shows an example of a plunging fold with flat hinges that give straight structure contours. However, the fold plunges so that the structure contours are not parallel, as illustrated. Complete the structure contours and extend them to the edges of the map. Determine the dips
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FIGURE 10.6 Example of nonplunging folds; see text.
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FIGURE 10.7 Example of plunging, angular antiform; see text.
and strikes at locations A–D. From the spacing of the structure contours along the hinge line (=axial trace) determine the plunge angle of the major fold. Draw a cross-section from L (Left) to R, extending the geology below and above
the topographic surface. The hinge of the major fold crops out on the hillside to the right of A; its shape gives a clue to the curvature of the hinge in cross-section, i.e., it should not be sharply angular. Note the presence of minor folds near
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FIGURE 10.8 Plunging folds are difficult to accurately represent in cross-section. The map (a) may be viewed down plunge (b) to reveal the true plunge profile. This may be constructed by redrafting the map on a compressed grid.
A, B, and C and their change in symmetry across the hinge of the major fold. Can you represent these in the crosssection? Note that the cross-section does not reveal a true plunge profile. A special construction as follows is required to reveal the profile. Figure 10.8 shows how we may reveal the true appearance of a fold’s plunge profile. (a) is a map of the folds with a 30° plunge (shown by the dip at the hinge). If one tilts the map and views the fold obliquely (b) at a 30° angle the true shape of the fold is revealed. This may be drawn by gridding the map (a) and redrawing the fold with a compression factor of sine (30°) as in (c). (c) is the true plunge profile. Now return to Figure 10.5 and produce a plunge profile of those folds. Figure 10.9 introduces greater complexity. Major folds do not plunge but they are partly concealed by an unconformable cover. Cleavage symbols show the orientation of a vertical axial plane cleavage. First, mark the unconformities (trace them out). Determine the dips of strata at a–d and show them on the map. Mark the unconformity. Construct structure contours for the folded beds below the unconformity (there is only one bedding plane that can be used). Locate the axial traces of the folds and mark them on the map, beneath the unconformity. They are located between the central structure contours for the folded bed.
Draw a cross-section LR with L on the left, extend the geology above and below the topography. Make sure you calculate the apparent dips of strata correctly in the section using strike lines as per exercise (Figures 7.1 and 7.2). Can you draw the subcrop of the bedding plane beneath the unconformity? Figure 10.10 provides a light relief from the previous exercises. Complete the map using the information shown in the EW sections at the top and bottom of the map. Note strike-and-dip symbols provide clues on the map. Note that the fold is not angular but gently curved. Figure 10.11 requires you to complete the geology north of the fault but first, determine dips and strikes at A through E. Construct structure contours south of the fault, adjust them to determine the elevation of structure contours north of the fault (for simplicity, the throw is 100 m). Construct and label the structure contours north of the fault and complete the geology north of the fault. Determine the borehole logs at W and Y (i.e., the vertical sequence of strata at those locations.) Draw a cross-section left to right, LR, carefully calculating the apparent dips of strata using the method of Figure 7.2. Figure 10.12 reveals upright folds with a fault history. Determine structure contours for the fault, determine its dip and construct structure contours for the stratum north of the
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FIGURE 10.9 Horizontally plunging (i.e., nonplunging) folds, partly concealed by an unconformity. Locate the axial traces of all folds and complete cross-section.
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FIGURE 10.10 Complete the map of the plunging, curved fold, from the cross-sections.
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FIGURE 10.11 Complete the map of the simple, nonplunging fold north of the fault. Use structure contours from the south side of the fault to map in the geology north of the fault.
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FIGURE 10.12 Locate the fold axial traces and determine the throw on the fault using carefully labeled structure contours.
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fault. Construct structure contours south of the fault and determine the throw and nature of the fault. Draw a section left to right, LR. Explain the nature of displacement of the dikes. Mark the axial traces of the folds; they are straight lines since the folds are upright and parallel to the axial planar cleavage. Figure 10.13 shows a series of simply folded beds beneath an unconformable cover. Trace out the unconformity, determine the dip and strike of the unconformity and overlying strata. Construct structure contours for one horizon of the folded strata. Locate the axial trace of the fold(s) and indicate the dip and strike of the flanks. Do the folds plunge? Construct the subcrop of the folded sandstone horizon as per exercise (Figures 8.4 and 8.5). Construct a cross-section LR with L on the left. How does the angle of folding differ in this section from the true hinge angle? The vergence (asymmetry) of minor folds is shown at the western edge of the area. Can you complete a suitable symbol for vergence at a–d? Subsequent exercises will show minor folds and axial plane cleavage associated with the major folds on the maps. Commonly, minor folds are referred to as having a “Z” or “S” geometry on the flanks of a major fold (and an “M” shape on the hinge). An example is shown in Figure 10.14(b), where the minor folds flank an antiform. However, the view of the minor folds depends very much on the surface orientation; in (a), the folds both yield an S and a Z geometry but the interpretation is unique, an antiform lies to the south. For this reason, it is better to note the interpretation of the asymmetry as in (b), using a solid circle for the N side of the antiform and an open circle for the S side of the antiform. The map of the road section in (c) shows how the ambiguity may be resolved. Other exercises will show axial plane cleavage; this is an important minor structure used in mapping (Figure 10.15). Also known as Pumpelly’s rule, it basically states that the cleavage is, as a general rule, closer in orientation to the axial plane than the flanks of the fold. Thus, given outcrops P and Q, the only solution is that (at least one) antiform lies between the outcrops. The mechanical basis for this is that even at low metamorphic grades, the compression causing folding simultaneously causes metamorphic reactions that align minerals in the most compliant orientation (i.e., micas parallel to axial plane). (b) shows the interpretation of (a). (c) shows the interpretation of cleavage bedding angles on the left hand flank of an antiform and an overturned antiform. Figure 10.16 introduces a fold with overturned limbs. The folds do not plunge so that the structure contours appear as approximately straight lines. (Some symbols for bedding indicate the strike direction.) What is the trend of the fold indicated? Draw structure contours on the quartzite–slate boundary, starting at the left hand edge of the page. Mark dip-and-strike symbols as you go and indicate the dip angles. You will first define an antiform trace that will curve in a
Understanding Geology Through Maps
similar fashion to one shown on the left hand side of the map. However, it construction may be postponed at this stage. Complete the structure contours to define a synform between the two antiforms; note that the vergence symbols agree with the shapes of minor folds. Draw a section LR (left to right). Now draw in the axial traces of the folds. However, this is not a trivial procedure. Since the fold axial planes dip to the left, their axial traces have a curvature that must be in sympathy with topography. You can achieve this by drawing structure contours for the axial planes and construct the axial traces from their intersection with topography. Finally, note the vergence of the minor folds indicated for the fold in the slate near its boundary with the quartzite. In a similar fashion, draw the vergence of minor folds at the four locations marked with a heavy circle on the eastern antiform. Also determine the mean dip and strike at those locations and indicate it on the map. (Note the use of an inverted bed symbol.) Figure 10.17 introduces overturned folds with cleavage and a fault. Locate axial traces of folds and indicate the dip of strata at various locations. Note the axial planar schistosity is a clue to the dip of the axial planes. Determine the dip and strike of the fault and its sense and amount of throw. What kind of fault is it? Draw a cross-section LR from left to right. Describe the structural history. On the cross-section, indicate the vergence (asymmetry) of minor folds at six locations. Can this be shown on the map? Figure 10.18 shows a plunging, reclined (reclined = axial plane dips) fold of a sandstone bed. Construct the structure contours for one horizon on each flank of the fold. Where these meet at the same elevation, they will locate the hinge of the fold. Mark the dip and strike of the flanks on the map and the trend and plunge of the hinge. Draw a cross-section of your choice to reveal the nature of the fold. Construct the sinuous course of the axial trace across the map (since it dips it must be influenced by topography as in Figure 10.16). What is the true interlimb angle in the field and what is the apparent interlimb angle in your section? Using the symbol for the vergence of minor folds indicate the expected vergence on the map at locations a–d taking account of topography and major fold vergence. What is the trend and plunge of the major fold? Show this on the map with an arrow. Figure 10.19 presents overturned but not plunging folds of a single horizon. Some field symbols show orientations of schistosity, bedding, and overturned bedding. Determine the axial traces of the folds as accurately as possible and draw a section LR from left to right. Using the symbol for the vergence of minor folds, indicate the expected vergence on the map at locations a–d taking account of topography and major fold vergence. Figure 10.20 shows the structure of a complicated area. Note the minor fold symmetry and the orientation of beds. Describe the structural history as fully as possible using constructions to show your understanding. Indicate the dip angles at the locations of the dip-and-strike symbols.
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FIGURE 10.13 Construct structure contours for the folds, complete the subcrop of the folded sandstone beneath the unconformity. Draw a cross-section LR from left to right.
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FIGURE 10.14 Subsequent maps will show minor folds on the flanks of the major folds. The asymmetry of the minor folds changes across the axial trace of the major fold but care must be taken to note the orientation of the surface on which the minor fold crops out. Thus, merely noting “S” or “Z” symmetry will not usually suffice. See text.
FIGURE 10.15 Overturned folds verging to the West. Using structure contours, complete the cross-section left to right (LR). Then, interpolate structure contours for the axial planes (their dip is intermediate between the limbs) and map in the axial traces. Add more minor fold vergences and indicate the orientation of axial plane cleavage.
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FIGURE 10.16 Some subsequent maps will show cleavage, axial planar to the major folds. The relative angles of cleavage and bedding (a) bracket the locations of major folds because cleavage is generally closer to the orientation of the axial plane (b).
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FIGURE 10.17 Isolate the folds using structure contours and locate their axial traces. Determine the dip and strike of the fault and the nature of it motion from the structure contours of the beds. Draw a cross-section left to right (LR). Add more indications of axial plane schistosity and estimate their dips.
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FIGURE 10.18 Plunging, overturned fold. To understand this fold and construct it axial plane, it is necessary to draw carefully labeled structure contours on one side of the bed. Indicate the dips of the flanks and the plunge of the fold. Mark in the axial trace bearing in mind its dip is midway between that of the flanks.
Figure 10.21 is a relatively simple review question. Determine the nature and motion of the fault, the location of fold(s), and the geological history. Locate the axial trace(s) of folds since the folds are upright, the traces are straight. A more complex question is to fix the vergence of minor folds, using the given symbol, at locations a–d. Be careful to take account of the vergence of the major fold and of the topography.
EXCERPTS SIMPLIFIED FROM PUBLISHED GEOLOGICAL SURVEY MAPS In all of the maps, the area is sufficiently large that folded strata are recognizable without topographic interpretation. Thus, topography is not shown and the use of structure contours is not required. However, in some maps, such as the Bristol sheet, very gently dipping strata have tortuous outcrops due
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FIGURE 10.19 Overturned fold. Draw structure contours carefully for one bedding plane. Determine the dips of the beds and the plunge of the fold. Indicate the axial trace.
Chapter | 10 Folds
FIGURE 10.20 Overturned fold. Determine the geological history of the map and produce the cross-section.
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FIGURE 10.21 Describe the geological history of the area, including details of the fault and fold. Produce a cross-section LR.
Chapter | 10 Folds
to topographic effects. Their dendritic pattern indicates their superficial and horizontal nature. This area is affected by preTriassic folding due to the Variscan (=Hercynian) orogeny.
Bristol District SW, UK 1. T he map has a scale that dwarfs the topographic relief, however, the very flat-lying strata do show some correlation with topography; namely, they follow dendritic drainage patterns (Figure 10.22). 2. On the map, indicate approximately, with pencil, the axial traces of any major folds. Use the appropriate symbol to distinguish antiforms and synforms. Use appropriate terms to describe the tightness, style, and orientations of the folds (e.g., closed, reclined, S-plunging, verging west; verging = axial plane leaning toward). 3. Draw an N–S cross-section along the section line drawn near the left margin of the map. Keep south on the left-hand side. Indicate the orientations of strata to some depth so that your interpretation is clear. Keep the section true scale, only then may you use dip angles from the map directly in the section. However, remember that if strata do not dip in a direction parallel to the section, they will show reduced apparent dips (recall Figures 7.1 and 7.2). 4. Establish a stratigraphic column that includes all geological events of significance. Identify the relative age and type of the principal stratigraphic discontinuities within the column. Also, indicate within the stratigraphic column where folds and different kinds of fault may have occurred. 5. Identify how many systems of faulting are present, by type and by orientation. What are the principal senses of motion of each group you identify? What are their ages of the groups identified, relative to folds and stratigraphic breaks?
East Falkland Islands, South Atlantic East and West Falkland are parts of the ancient Gondwana continent, which were located close to the south pole during the Carboniferous, witnessed by the formation of the Fitzroy tillite (Figure 10.23). A tillite is a lithified till. Subsequent folding and overthrusting occurred followed by shallow water deposits of the Lafonia Group. The geology is very similar to parts of South Africa to which the Falklands were adjacent in Gondwana land. 1. T his area is even larger than the previous map so that topography plays no part in the distribution of lithological boundaries. 2. Draw a cross-section LR with L on the left showing the orientation of the strata. Note that some younging arrows point in the opposite direction to the dip of the beds, meaning that the beds (and fold limbs) are overturned.
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North West Cyprus This large area of the NW part of the island of Cyprus in the eastern Mediterranean exposes an ophiolite, of Cretaceous age (mantle sequence approximately 88 Ma) (Figure 10.24). The mantle sequence of gabbros in the south center of the map forms the high point but the distribution of lithologies is, in the first instance, not topographically controlled at this scale. Rather, the mantle sequence and surrounding dike complex form an elongate dome running ENE–WSW. The doming is due to N–S shortening and perhaps due to hydration and volume increase of the mantle rocks at depth. The Paleogene limestone sequence drapes over the ophiolite complex and represents the original ocean floor sedimentary cover. NE of Morphou, a different tectonic terrane occurs representing a young fold and thrust sequence. In this case, different dip-and-strike symbols are used for the primary layering of sedimentary and volcanic igneous rocks. North of the ophiolite, east of Morphou, and to the north coast, a fold and thrust belt is exposed. What is its sense of vergence? Draw a cross-section, ignoring topography, from L (left) to R (right).
Pierre Greys Lakes, Alberta This map of tight folding covers a large area so that lithology controls the fold outcrop pattern not topography (Figure 10.25). The area was folded and metamorphosed during the Laramide Orogeny, approximately 70–40 Ma ago. Note that the lithologies are distributed in a regular elongate “grain” since there is a single tight episode of folding. Draw a cross-section of this fold and thrust belt from L to R, with R being on the side toward which thrusting and overfolding occurred. To which direction do the folds verge (vergence = direction to which axial planes lean)? Which is the direction of overthrusting?
Pembroke East, S. Wales Note that the symbol used for overturned strata is now antiquated (Figure 10.26). The overturned folds have low plunges and were formed in the Hercynian (=Variscan) orogeny, a prePermian event. Add axial traces to the principal folds. Note also the pronounced “grain” to the geology as in the previous figure. At this scale, topographic relief does not influence the outcrop pattern. Draw a cross-section LR with L on the left. Some faults are thrusts. Which faults are thrusts and what is the direction of their overthrusting? In which direction, do the folds verge? (Is that apparent from the cross-section?)
Gananoque Area, Ontario This area has tightly and isoclinally folded gneiss, deformed in the Grenville orogeny at approximately 1100 Ma (Figure 10.27). These rocks are nonconformably overlain by
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FIGURE 10.22 Major folds beneath an unconformity in the Bristol area of UK.
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FIGURE 10.23 East Falklands geology showing major regional folds.
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FIGURE 10.24 West Cyprus showing a major dome of an ophiolite in the south and a fold and thrust belt in the north.
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FIGURE 10.25 Pierre Greys Area, Canada showing a fold and thrust belt.
nondeformed Ordovician sandstone and dolomite. Trace out the unconformity. High grade of metamorphism and intense heterogeneous strain obscures the stratigraphic order of the Pre-Cambrian rocks. The orientation of the first schistosity (S1) is given by the dip and strike symbols. This is axial planar to the first phase of folding (F1). You will see that a second phase of folding (F2) has deflected the earlier folds, after which gneissose granite was emplaced. Although the folding is tight or isoclinal, the “grain” to the geology is poorly developed because the phases of folding interfere with one another and are on a similar scale. Label some fold axial planes F1 and F2 as appropriate. Tabulate a geological history for the area and draw a sketch cross-section L (left) to R. Figure 10.28 shows an area of low topographic relief in England. Describe the fold and its orientation and construct a plunge profile as explained in Figure 10.8. What is the significance of the orientation of the cleavage–bedding intersection lineations? Slickensides are grooves or growth fibers related to bedding plane slip and are generated as bedding planes slide past one another during folding.
Figure 10.29 shows a map of the Loch Awe Syncline (LAS) and two adjacent minor folds. The Loch Awe Syncline is a major fold in SW Scotland affecting the Eocambrian Dalradian Series. Her, the Dalradian is unconformably overlain by Devonian lava. Draw a cross-section left to right (LR) and also produce a plunge profile of the structure. Is the Loch Awe Syncline overturned? Is its cleavage exactly axial planar? (See Figure 10.15.) Figure 10.30 maps the contact between the Dalradian Supergroup and the older Moine Supergroup in Donegal, Ireland. The vergence of minor folds is shown by the black dot/ open circle convention of Figure 10.14; larger minor folds are shown by the sinuous path of layering (foliation) which is termed an L–S fabric (L = lineation; S = schistosity). The contact between the Dalradian and Moine is a tectonic slide that is a shear zone with a normal sense of displacement. Draw a plunge profile of the folding using the average mineral lineation as the plunge of the folds. The folds in Figure 10.30 and some earlier diagrams show curvature of the fold hinges. Such folds are termed
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FIGURE 10.26 East Pembrokeshire, UK, showing folds and thrusts.
periclinal folds (or whaleback folds) and they produce variations in axial plunge and the orientation of cleavage– bedding intersection lineations. Figure 10.31(a) shows the initiation of such a fold due to heterogeneous flattening across the axial plane. A more extreme case is shown in (b) in which case the folds would be called sheath folds; younging directions are highly dispersed. An actual example of natural sheath folds is shown in (c) from Atikokan in N. Ontario. The section is approximately 10 km long, very well exposed, and abundant graded bedding and cleavage bedding relations reveal the structure as shown. Figure 10.32 is a hypothetical map of folds with axial plane cleavage. Draw a cross-section LR showing the attitude of minor structures (folds and cleavage) and then produce a plunge profile. Figure 10.33 is a famous cross-section of the SW Scottish Highlands mapped by Sir Edward Bailey (1934).
The section shows an isoclinally folded stratigraphy, but the isoclinals F1 recumbent folds have been refolded by upright F2 folds. First, trace in pencil the axial traces of the F1 and F2 folds across the section. Second, in the box below the cross-section make a sketch map of the geology in the section. Add symbols to show the orientation of bedding and first cleavage. The structural facing is the component of young projected onto the axial plane first cleavage; it yields sense of stratigraphic sequence from a single outcrop. This area of negligible topographic relief occurs near Shebandowan, N. Ontario and was mapped by W. M. Schwerdtner (1984) (Figure 10.34). It reveals a pluton that has been refolded as revealed by the orientations of deflected L and S fabrics. Draw a plunge profile of the F2 fold and mark on it the orientation of refolded S1. Are the minor folds at the hinge of the pluton’s major fold F1 or F2?
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FIGURE 10.27 Gananoque East area, Canada, showing multiply folded and metamorphosed rocks of the Canadian Shield (Map after Hewitt, 1964).
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FIGURE 10.28 Plunging anticline of Pre-Cambrian slates in Eastern England. Produce a plunge profile (see text).
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FIGURE 10.29 Overturned synform in the south (Loch Awe Syncline, SW Scotland.) Cleavage–bedding relationships and way up help reveal the structure (draw a section LR).
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FIGURE 10.30 Tightly folded high-grade schists and psammites, Donegal. Minor fold asymmetry (vergence) reveals the major structure.
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FIGURE 10.31 Examples of kilometer size folds from Atikokan, N. Ontario. The complex fold forms are due to severe plastic deformation progressing from the upper diagrams (a and b) to the final state in the lower diagram (c).
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FIGURE 10.32 Hypothetical folding. Draw a plunge profile.
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FIGURE 10.33 Cross-section of part of SW Scottish Highlands by E. Bailey (1934). The area shows two major fold episodes. Mark the axial traces of the F1 and F2 folds and complete a sketch map in the box below.
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FIGURE 10.34 Map of NW Ontario by W. M. Schwerdtner showing two major phases of folding.
Folds Chapter Outline Excerpts Simplified from Published Geological Survey Maps Bristol District SW, UK East Falkland Islands, South Atlantic
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When a geology student confronts a geological map for the first time, the geological boundaries may appear as a bewildering pattern. Soon we learn to recognize that the contortions and bends of boundaries are due the topographic surgery of the solid geology, especially if the map is at a small scale. Even a horizontally bedded sequence shows a complex outcrop pattern; it follows the contortions of the topographic contours precisely. Why is it that we are so comfortable with topographic maps and topographic contours yet initially perplexed when confronted with a geological map? Like the pioneer, William Smith, we learned to interpret the interference of geological layers with the topography in order to understand their relative ages. Perhaps by now we realize Smith’s genius. To paraphrase Charles Darwin, we do not understand simply by looking or sketching; we understand truly when observations are perceived by the “inner eye of reason”. We often take important simple ideas for granted; but remember that before someone introduced them there was usually ignorance or misunderstanding. So far, we discussed strata whose map patterns are sinuous due to topography. We understood in those examples that the strata were essentially planar and not actually contorted. Now we meet cases where the strata are contorted; they exhibit folds, undulations of the layers due to tectonic movements that usually commence with the process called buckling by structural geologist. Other processes, usually shortening, often subsequently amplify the buckles. Bucking has the attractive property that the undulations of layers have an approximately regular waveform as long as the layers have constant thickness and uniform lithology. Thus, geologist will talk of the wavelength and amplitude of folds, as though they were sinusoidal waveforms in physics. Of course, such precision is lacking in nature Understanding Geology Through Maps. http://dx.doi.org/10.1016/B978-0-12-800866-9.00010-7 Copyright © 2014 Elsevier Inc. All rights reserved.
North West Cyprus Pierre Greys Lakes, Alberta Pembroke East, S. Wales Gananoque Area, Ontario
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and some folding processes produce highly irregular nonperiodic undulations of layers, even if they are of uniform thickness and lithology (e.g., disharmonic folds, ptygmatic folds, etc. in high-grade rocks). Since any folds produce different changes in orientation in different parts of the bedrock they are considered as examples of heterogeneous strain. Homogeneous strain occurs where straight lines remain straight and parallel lines remain parallel. Homogeneous strain is rather rare and usually only approximated at some certain scale. Every rock is heterogeneously strained at some scale or other. Folds may occur in soft sediment due to slumping. More commonly, we recognize them as the result of tectonic movements in sedimentary rock, in metamorphic rock of every grade, and in some layered igneous rocks. Folding is a common response of rocks and sediments to motions slower than those involved with faulting and usually with stresses that are much lower than those causing faulting. Generally, they result from steady-state metamorphic flow that also produces schistosity or cleavage approximately parallel to the axial planes. A fabric lineation may also be produced parallel to the maximum extension direction that is normally at a high angle to fold axes. A fold is a wrinkle whose alignment or axis is perpendicular, or nearly so, to the shortening direction. Axes are not necessarily perpendicular to the maximum compressive stress since folds evolve slowly; in contrast, a state of stress is an instantaneous phenomenon. Folds accumulate increments of strain over a long period to produce a state of heterogeneous finite strain. Thus, as explained in Chapter 9, finite strain structures such as folds cannot be related to stress axes, which are instantaneous changing phenomenon (Figure 9.6). 141
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Calculations suggest that cliff-sized folds along the Devonshire coast of England took hundreds of thousands of years to form. On the other hand, faults may propagate at the velocity of sound in rock (several kilometers per second) and movements on them may be measurable in meters per second. Huge flat-lying overfolds (recumbent folds or nappe folds) form in metamorphic rocks with stress differences between minimum and maximum compression of less than 100 bars (10 MPa) within a few million years. In contrast, brittle failure causing fault motion may require more than 1 kbar (>100 MPa) differential stress and occur in seconds. The shapes that folds may take and the orientations of those folds give rise to a wide range of geometric possibilities that we may meet. In this chapter, we shall deal with folds at a rather simple level although the description of folds in the diagrams is somewhat more encompassing than we need and is given for your future reference. Even then, the simplification is made that the folds are regular geometrical forms; in particular, that they preserve the same degree of folding along their hinge-line length. Initially, we will consider only angular (chevron) folds since these have planar limbs (Figure 10.1(a)). Folds with curving hinges and curved limbs are considered later but a simple classification based on interlimb angle and axial plane dip is given in Figure 10.1(b and c). Initially also, we consider only folds that have a horizontal plunge, i.e., the fold axis is not tilted. Initially also, we restrict ourselves to upright folds, ones in which the axial plane is vertical. Thus, after reviewing the diagrams, we will first tackle the simplest fold map construction where the folds are 1. a ngular (=flat limbs, sharp hinge), 2. upright (=axial plane of symmetry vertical), and 3. nonplunging (=axis or hinge line is horizontal). These diagrams will be reviewed but here we address the question that may have arisen earlier in your minds. Planar, nonfolded strata crop out along contorted patterns, dictated by topography. How then will we determine from map view, if the sinuous outcrop patterns are due to folding and topography or solely due to topographic effects? This problem is not as great as it may seem. The following steps are useful. 1. C an a bend of strata be recognized anywhere? a. Consider the case where the topography is negligible, as in most regional maps such as scales of 1:100,000, 1:200,000, etc. In these cases, any substantial bends in a stratum will indicate the hinges of plunging folds. Where fold hinges are horizontal or plunge very gently, map view may not reveal the closure or curvature of the fold hinge. However, in that case, sequences of strata (e.g., A–B–C) will repeat in mirror image order (i.e., C–B–A) across the map.
Understanding Geology Through Maps
b. Where topography is a consideration, e.g., at scales of 1:50,000 or 1:10,000, reading the sense with which beds “climb” or “descend” slopes will reveal the presence of a fold. For example, even a fold with a horizontal axis will expose the closure (bend) of the hinge region where the fold axis meets a topographic slope. Inspect the trend of the strata; this must correspond broadly to their strike and broadly to the fold axis trend. Where the fold meets a slope at a high angle, the same bed may change elevation as it crosses the slope to form a downslope bend (synform) or an upslope bend (antiform). This is a robust mapreading approach that simply detects the presence of a fold by inspection. It rarely reveals fundamental information concerning the shape or orientation of the fold since the map view contortions of a folded bed are dependent of the topography. However, we shall see that simple constructions reveal quantitative information on the fold’s shape and orientation. Eventually, the geologist learns to perform similar qualitative interpretations mentally. 2. Is the stratigraphic sequence repeated in two nearly opposite directions? A fold necessarily bends the strata so that the two flanks (limbs) tilt the strata in nearly opposite directions. When exposed by erosion, the sequence of the beds crops out in mirror image on either side of the fold’s axis. For example, the beds get younger away from the core of an anticline; the beds get older away from the core of a syncline. Consider a sequence of beds, A is older than B which is older than C. Now, inspect the map; does the sequence A → B → C occur in one direction (say, younging eastwards) in one part of the map and does it then occur younging eastwards C ← B ← A, in another part of the map? This would indicate that the strata dipped in opposite directions due to folding. 3. Are dip-and-strike symbols present? On some maps, dip-and-strike symbols will provide the obvious clue to the presence of folds. An antiform separates obviously opposed dip directions and face-to-face dip directions occur on either flank of a synform. A further clue to the presence of folds may be provided by antiparallel, or nearly opposed, younging arrows, Y [younger beds at the foot of the letter symbol], mainly on maps of very complexly deformed rock. However, some caution may be required in the use of structural symbols, depending on the scale of the map and the size of the folds. For maps that cover very large areas (1:50,000 and 1:100,000), the symbol must be very carefully placed to avoid misleading the map reader since actual outcrops are smaller than the symbol. It should also be remembered that the symbol may cover a huge ground area, of several hundred square meters. A common convention is that the tip of the symbol locates the site of the observation. This does not help very much in many cases; I have
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FIGURE 10.1 Classification of simple fold forms. Most natural folds have much more complex curving shapes (a) (b) and (C) identified in drawing.
seen regional maps (1:100,000 and larger) on which the area covered by the symbol is so large that it obscures the area in which the fold crops out. This is generally the reason why regional maps omit all small-scale structure symbols of any kind. Such large-scale maps also present a further difficulty; topographic relief is usually so
small in comparison to the map scale that we cannot use topography–geology relations to deduce reliable geological relations such as dips and sequence. Such largescale maps tend to become diagrams that only show geographical distributions of rocks. Three-dimensional interpretation requires some further skills.
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4. C onstruct stratum contours! As with faults, the only reliable way to determine the presence, nature, and effects of folds is to construct stratum contours (Figure 10.2). This technique works where we have small scales and topographic contours. Structure contours will trace out the form of a bed as if it cropped out on a hypothetical horizontal plane (or mine plan). Clearly, the same stratum contour (e.g., sandstone–limestone, 350 m) will appear twice for each fold, once on each fold limb. With good “book keeping”, by labeling each stratum contour carefully and working in erasable pencil, we may interpret such maps uniquely. A common elementary confusion is that when the intersections of topography and a bedding plane are determined, upon first sight there may appear to be ambiguity in the way they are connected. The correct of possibilities will be parallel to the respective flanks of the fold. The alternative set of misconstructed stratum contours will cut across the symmetry of the fold and be inconsistent with topography. Recall earlier that we learned that a geological boundary appears only at the topographic surface if is justified by the intersection of a topographic contour and stratum contour of the same elevation. 5. Where folds plunge (Figure 10.3), the stratum contours from opposing flanks will meet at the hinge
Understanding Geology Through Maps
(Figure 10.3(b and c)). The separation of the stratum contours along the axial trace permits the plunge of the fold to be determined. This is equivalent to determining the dip of the hinge of the fold along the axial trace (Figure 10.4). Some natural structures flag the geometry of the major fold. For example, cleavage or schistosity is commonly parallel to the axial planes of fold (Figure 10.4(a)). In a similar fashion, minor folds on the flanks of major folds will define the plunge of the major fold (Figure 10.4(b)). Note that the asymmetry of the minor folds changes from one major flank to the other; commonly, these are termed “S” and “Z” asymmetry folds. However, their exact appearance on the map also depends on the slope of the topographic surface on which they appear; this will become apparent in later exercises (e.g., Figure 10.14). Natural folds are not as simple or “perfect” as the examples in the preceding diagrams. An example of a natural fold is mapped in Figure 10.5. Note that the main synform is accompanied by two minor folds on its southern flank. However, these folds do not persist for a long distance along their axial traces. Such folds are termed “disharmonic” as minor axial traces eventually disappear, merge, or are faulted out.
FIGURE 10.2 Isolation of folds (in this case an antiform) using structural contours. (a) Simple antiform (b) Same with stratum contours (c) Map view of structure contours.
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FIGURE 10.3 (a) A simple plunging angular antiform (b) structure contours (c) map view.
FIGURE 10.4 A rounded antiform; note the curving structural contours. (a) three-dimensional structure (b) map.
Some simple exercises with folds illustrate these concepts. First, in Figure 10.6, we meet upright folds without plunge; structure contours will be parallel and straight. Determine the stratigraphic order by examining
topographic slopes (shale and sandstone are illustrated). Determine the dip angles at the locations marked with dipand-strike symbols. Using labeled stratum contours, determine the presence of any folds. Mark the axial traces of
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FIGURE 10.5 An example of a natural fold, from northern England. Note the variation of limb thicknesses and the fact that the minor folds are disharmonic, i.e., they vary in amplitude along their axial planes, causing them to disappear or merge.
the folds following the examples given. Draw a true-scale cross-section LR with L on the left, extrapolate the geology above and below the topography. The cross-section is perpendicular to the fold axes and thus it will show a true plunge profile.
Figure 10.7 shows an example of a plunging fold with flat hinges that give straight structure contours. However, the fold plunges so that the structure contours are not parallel, as illustrated. Complete the structure contours and extend them to the edges of the map. Determine the dips
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FIGURE 10.6 Example of nonplunging folds; see text.
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FIGURE 10.7 Example of plunging, angular antiform; see text.
and strikes at locations A–D. From the spacing of the structure contours along the hinge line (=axial trace) determine the plunge angle of the major fold. Draw a cross-section from L (Left) to R, extending the geology below and above
the topographic surface. The hinge of the major fold crops out on the hillside to the right of A; its shape gives a clue to the curvature of the hinge in cross-section, i.e., it should not be sharply angular. Note the presence of minor folds near
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FIGURE 10.8 Plunging folds are difficult to accurately represent in cross-section. The map (a) may be viewed down plunge (b) to reveal the true plunge profile. This may be constructed by redrafting the map on a compressed grid.
A, B, and C and their change in symmetry across the hinge of the major fold. Can you represent these in the crosssection? Note that the cross-section does not reveal a true plunge profile. A special construction as follows is required to reveal the profile. Figure 10.8 shows how we may reveal the true appearance of a fold’s plunge profile. (a) is a map of the folds with a 30° plunge (shown by the dip at the hinge). If one tilts the map and views the fold obliquely (b) at a 30° angle the true shape of the fold is revealed. This may be drawn by gridding the map (a) and redrawing the fold with a compression factor of sine (30°) as in (c). (c) is the true plunge profile. Now return to Figure 10.5 and produce a plunge profile of those folds. Figure 10.9 introduces greater complexity. Major folds do not plunge but they are partly concealed by an unconformable cover. Cleavage symbols show the orientation of a vertical axial plane cleavage. First, mark the unconformities (trace them out). Determine the dips of strata at a–d and show them on the map. Mark the unconformity. Construct structure contours for the folded beds below the unconformity (there is only one bedding plane that can be used). Locate the axial traces of the folds and mark them on the map, beneath the unconformity. They are located between the central structure contours for the folded bed.
Draw a cross-section LR with L on the left, extend the geology above and below the topography. Make sure you calculate the apparent dips of strata correctly in the section using strike lines as per exercise (Figures 7.1 and 7.2). Can you draw the subcrop of the bedding plane beneath the unconformity? Figure 10.10 provides a light relief from the previous exercises. Complete the map using the information shown in the EW sections at the top and bottom of the map. Note strike-and-dip symbols provide clues on the map. Note that the fold is not angular but gently curved. Figure 10.11 requires you to complete the geology north of the fault but first, determine dips and strikes at A through E. Construct structure contours south of the fault, adjust them to determine the elevation of structure contours north of the fault (for simplicity, the throw is 100 m). Construct and label the structure contours north of the fault and complete the geology north of the fault. Determine the borehole logs at W and Y (i.e., the vertical sequence of strata at those locations.) Draw a cross-section left to right, LR, carefully calculating the apparent dips of strata using the method of Figure 7.2. Figure 10.12 reveals upright folds with a fault history. Determine structure contours for the fault, determine its dip and construct structure contours for the stratum north of the
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FIGURE 10.9 Horizontally plunging (i.e., nonplunging) folds, partly concealed by an unconformity. Locate the axial traces of all folds and complete cross-section.
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FIGURE 10.10 Complete the map of the plunging, curved fold, from the cross-sections.
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FIGURE 10.11 Complete the map of the simple, nonplunging fold north of the fault. Use structure contours from the south side of the fault to map in the geology north of the fault.
Chapter | 10 Folds
FIGURE 10.12 Locate the fold axial traces and determine the throw on the fault using carefully labeled structure contours.
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fault. Construct structure contours south of the fault and determine the throw and nature of the fault. Draw a section left to right, LR. Explain the nature of displacement of the dikes. Mark the axial traces of the folds; they are straight lines since the folds are upright and parallel to the axial planar cleavage. Figure 10.13 shows a series of simply folded beds beneath an unconformable cover. Trace out the unconformity, determine the dip and strike of the unconformity and overlying strata. Construct structure contours for one horizon of the folded strata. Locate the axial trace of the fold(s) and indicate the dip and strike of the flanks. Do the folds plunge? Construct the subcrop of the folded sandstone horizon as per exercise (Figures 8.4 and 8.5). Construct a cross-section LR with L on the left. How does the angle of folding differ in this section from the true hinge angle? The vergence (asymmetry) of minor folds is shown at the western edge of the area. Can you complete a suitable symbol for vergence at a–d? Subsequent exercises will show minor folds and axial plane cleavage associated with the major folds on the maps. Commonly, minor folds are referred to as having a “Z” or “S” geometry on the flanks of a major fold (and an “M” shape on the hinge). An example is shown in Figure 10.14(b), where the minor folds flank an antiform. However, the view of the minor folds depends very much on the surface orientation; in (a), the folds both yield an S and a Z geometry but the interpretation is unique, an antiform lies to the south. For this reason, it is better to note the interpretation of the asymmetry as in (b), using a solid circle for the N side of the antiform and an open circle for the S side of the antiform. The map of the road section in (c) shows how the ambiguity may be resolved. Other exercises will show axial plane cleavage; this is an important minor structure used in mapping (Figure 10.15). Also known as Pumpelly’s rule, it basically states that the cleavage is, as a general rule, closer in orientation to the axial plane than the flanks of the fold. Thus, given outcrops P and Q, the only solution is that (at least one) antiform lies between the outcrops. The mechanical basis for this is that even at low metamorphic grades, the compression causing folding simultaneously causes metamorphic reactions that align minerals in the most compliant orientation (i.e., micas parallel to axial plane). (b) shows the interpretation of (a). (c) shows the interpretation of cleavage bedding angles on the left hand flank of an antiform and an overturned antiform. Figure 10.16 introduces a fold with overturned limbs. The folds do not plunge so that the structure contours appear as approximately straight lines. (Some symbols for bedding indicate the strike direction.) What is the trend of the fold indicated? Draw structure contours on the quartzite–slate boundary, starting at the left hand edge of the page. Mark dip-and-strike symbols as you go and indicate the dip angles. You will first define an antiform trace that will curve in a
Understanding Geology Through Maps
similar fashion to one shown on the left hand side of the map. However, it construction may be postponed at this stage. Complete the structure contours to define a synform between the two antiforms; note that the vergence symbols agree with the shapes of minor folds. Draw a section LR (left to right). Now draw in the axial traces of the folds. However, this is not a trivial procedure. Since the fold axial planes dip to the left, their axial traces have a curvature that must be in sympathy with topography. You can achieve this by drawing structure contours for the axial planes and construct the axial traces from their intersection with topography. Finally, note the vergence of the minor folds indicated for the fold in the slate near its boundary with the quartzite. In a similar fashion, draw the vergence of minor folds at the four locations marked with a heavy circle on the eastern antiform. Also determine the mean dip and strike at those locations and indicate it on the map. (Note the use of an inverted bed symbol.) Figure 10.17 introduces overturned folds with cleavage and a fault. Locate axial traces of folds and indicate the dip of strata at various locations. Note the axial planar schistosity is a clue to the dip of the axial planes. Determine the dip and strike of the fault and its sense and amount of throw. What kind of fault is it? Draw a cross-section LR from left to right. Describe the structural history. On the cross-section, indicate the vergence (asymmetry) of minor folds at six locations. Can this be shown on the map? Figure 10.18 shows a plunging, reclined (reclined = axial plane dips) fold of a sandstone bed. Construct the structure contours for one horizon on each flank of the fold. Where these meet at the same elevation, they will locate the hinge of the fold. Mark the dip and strike of the flanks on the map and the trend and plunge of the hinge. Draw a cross-section of your choice to reveal the nature of the fold. Construct the sinuous course of the axial trace across the map (since it dips it must be influenced by topography as in Figure 10.16). What is the true interlimb angle in the field and what is the apparent interlimb angle in your section? Using the symbol for the vergence of minor folds indicate the expected vergence on the map at locations a–d taking account of topography and major fold vergence. What is the trend and plunge of the major fold? Show this on the map with an arrow. Figure 10.19 presents overturned but not plunging folds of a single horizon. Some field symbols show orientations of schistosity, bedding, and overturned bedding. Determine the axial traces of the folds as accurately as possible and draw a section LR from left to right. Using the symbol for the vergence of minor folds, indicate the expected vergence on the map at locations a–d taking account of topography and major fold vergence. Figure 10.20 shows the structure of a complicated area. Note the minor fold symmetry and the orientation of beds. Describe the structural history as fully as possible using constructions to show your understanding. Indicate the dip angles at the locations of the dip-and-strike symbols.
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FIGURE 10.13 Construct structure contours for the folds, complete the subcrop of the folded sandstone beneath the unconformity. Draw a cross-section LR from left to right.
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FIGURE 10.14 Subsequent maps will show minor folds on the flanks of the major folds. The asymmetry of the minor folds changes across the axial trace of the major fold but care must be taken to note the orientation of the surface on which the minor fold crops out. Thus, merely noting “S” or “Z” symmetry will not usually suffice. See text.
FIGURE 10.15 Overturned folds verging to the West. Using structure contours, complete the cross-section left to right (LR). Then, interpolate structure contours for the axial planes (their dip is intermediate between the limbs) and map in the axial traces. Add more minor fold vergences and indicate the orientation of axial plane cleavage.
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FIGURE 10.16 Some subsequent maps will show cleavage, axial planar to the major folds. The relative angles of cleavage and bedding (a) bracket the locations of major folds because cleavage is generally closer to the orientation of the axial plane (b).
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FIGURE 10.17 Isolate the folds using structure contours and locate their axial traces. Determine the dip and strike of the fault and the nature of it motion from the structure contours of the beds. Draw a cross-section left to right (LR). Add more indications of axial plane schistosity and estimate their dips.
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FIGURE 10.18 Plunging, overturned fold. To understand this fold and construct it axial plane, it is necessary to draw carefully labeled structure contours on one side of the bed. Indicate the dips of the flanks and the plunge of the fold. Mark in the axial trace bearing in mind its dip is midway between that of the flanks.
Figure 10.21 is a relatively simple review question. Determine the nature and motion of the fault, the location of fold(s), and the geological history. Locate the axial trace(s) of folds since the folds are upright, the traces are straight. A more complex question is to fix the vergence of minor folds, using the given symbol, at locations a–d. Be careful to take account of the vergence of the major fold and of the topography.
EXCERPTS SIMPLIFIED FROM PUBLISHED GEOLOGICAL SURVEY MAPS In all of the maps, the area is sufficiently large that folded strata are recognizable without topographic interpretation. Thus, topography is not shown and the use of structure contours is not required. However, in some maps, such as the Bristol sheet, very gently dipping strata have tortuous outcrops due
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FIGURE 10.19 Overturned fold. Draw structure contours carefully for one bedding plane. Determine the dips of the beds and the plunge of the fold. Indicate the axial trace.
Chapter | 10 Folds
FIGURE 10.20 Overturned fold. Determine the geological history of the map and produce the cross-section.
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FIGURE 10.21 Describe the geological history of the area, including details of the fault and fold. Produce a cross-section LR.
Chapter | 10 Folds
to topographic effects. Their dendritic pattern indicates their superficial and horizontal nature. This area is affected by preTriassic folding due to the Variscan (=Hercynian) orogeny.
Bristol District SW, UK 1. T he map has a scale that dwarfs the topographic relief, however, the very flat-lying strata do show some correlation with topography; namely, they follow dendritic drainage patterns (Figure 10.22). 2. On the map, indicate approximately, with pencil, the axial traces of any major folds. Use the appropriate symbol to distinguish antiforms and synforms. Use appropriate terms to describe the tightness, style, and orientations of the folds (e.g., closed, reclined, S-plunging, verging west; verging = axial plane leaning toward). 3. Draw an N–S cross-section along the section line drawn near the left margin of the map. Keep south on the left-hand side. Indicate the orientations of strata to some depth so that your interpretation is clear. Keep the section true scale, only then may you use dip angles from the map directly in the section. However, remember that if strata do not dip in a direction parallel to the section, they will show reduced apparent dips (recall Figures 7.1 and 7.2). 4. Establish a stratigraphic column that includes all geological events of significance. Identify the relative age and type of the principal stratigraphic discontinuities within the column. Also, indicate within the stratigraphic column where folds and different kinds of fault may have occurred. 5. Identify how many systems of faulting are present, by type and by orientation. What are the principal senses of motion of each group you identify? What are their ages of the groups identified, relative to folds and stratigraphic breaks?
East Falkland Islands, South Atlantic East and West Falkland are parts of the ancient Gondwana continent, which were located close to the south pole during the Carboniferous, witnessed by the formation of the Fitzroy tillite (Figure 10.23). A tillite is a lithified till. Subsequent folding and overthrusting occurred followed by shallow water deposits of the Lafonia Group. The geology is very similar to parts of South Africa to which the Falklands were adjacent in Gondwana land. 1. T his area is even larger than the previous map so that topography plays no part in the distribution of lithological boundaries. 2. Draw a cross-section LR with L on the left showing the orientation of the strata. Note that some younging arrows point in the opposite direction to the dip of the beds, meaning that the beds (and fold limbs) are overturned.
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North West Cyprus This large area of the NW part of the island of Cyprus in the eastern Mediterranean exposes an ophiolite, of Cretaceous age (mantle sequence approximately 88 Ma) (Figure 10.24). The mantle sequence of gabbros in the south center of the map forms the high point but the distribution of lithologies is, in the first instance, not topographically controlled at this scale. Rather, the mantle sequence and surrounding dike complex form an elongate dome running ENE–WSW. The doming is due to N–S shortening and perhaps due to hydration and volume increase of the mantle rocks at depth. The Paleogene limestone sequence drapes over the ophiolite complex and represents the original ocean floor sedimentary cover. NE of Morphou, a different tectonic terrane occurs representing a young fold and thrust sequence. In this case, different dip-and-strike symbols are used for the primary layering of sedimentary and volcanic igneous rocks. North of the ophiolite, east of Morphou, and to the north coast, a fold and thrust belt is exposed. What is its sense of vergence? Draw a cross-section, ignoring topography, from L (left) to R (right).
Pierre Greys Lakes, Alberta This map of tight folding covers a large area so that lithology controls the fold outcrop pattern not topography (Figure 10.25). The area was folded and metamorphosed during the Laramide Orogeny, approximately 70–40 Ma ago. Note that the lithologies are distributed in a regular elongate “grain” since there is a single tight episode of folding. Draw a cross-section of this fold and thrust belt from L to R, with R being on the side toward which thrusting and overfolding occurred. To which direction do the folds verge (vergence = direction to which axial planes lean)? Which is the direction of overthrusting?
Pembroke East, S. Wales Note that the symbol used for overturned strata is now antiquated (Figure 10.26). The overturned folds have low plunges and were formed in the Hercynian (=Variscan) orogeny, a prePermian event. Add axial traces to the principal folds. Note also the pronounced “grain” to the geology as in the previous figure. At this scale, topographic relief does not influence the outcrop pattern. Draw a cross-section LR with L on the left. Some faults are thrusts. Which faults are thrusts and what is the direction of their overthrusting? In which direction, do the folds verge? (Is that apparent from the cross-section?)
Gananoque Area, Ontario This area has tightly and isoclinally folded gneiss, deformed in the Grenville orogeny at approximately 1100 Ma (Figure 10.27). These rocks are nonconformably overlain by
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FIGURE 10.22 Major folds beneath an unconformity in the Bristol area of UK.
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FIGURE 10.23 East Falklands geology showing major regional folds.
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FIGURE 10.24 West Cyprus showing a major dome of an ophiolite in the south and a fold and thrust belt in the north.
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FIGURE 10.25 Pierre Greys Area, Canada showing a fold and thrust belt.
nondeformed Ordovician sandstone and dolomite. Trace out the unconformity. High grade of metamorphism and intense heterogeneous strain obscures the stratigraphic order of the Pre-Cambrian rocks. The orientation of the first schistosity (S1) is given by the dip and strike symbols. This is axial planar to the first phase of folding (F1). You will see that a second phase of folding (F2) has deflected the earlier folds, after which gneissose granite was emplaced. Although the folding is tight or isoclinal, the “grain” to the geology is poorly developed because the phases of folding interfere with one another and are on a similar scale. Label some fold axial planes F1 and F2 as appropriate. Tabulate a geological history for the area and draw a sketch cross-section L (left) to R. Figure 10.28 shows an area of low topographic relief in England. Describe the fold and its orientation and construct a plunge profile as explained in Figure 10.8. What is the significance of the orientation of the cleavage–bedding intersection lineations? Slickensides are grooves or growth fibers related to bedding plane slip and are generated as bedding planes slide past one another during folding.
Figure 10.29 shows a map of the Loch Awe Syncline (LAS) and two adjacent minor folds. The Loch Awe Syncline is a major fold in SW Scotland affecting the Eocambrian Dalradian Series. Her, the Dalradian is unconformably overlain by Devonian lava. Draw a cross-section left to right (LR) and also produce a plunge profile of the structure. Is the Loch Awe Syncline overturned? Is its cleavage exactly axial planar? (See Figure 10.15.) Figure 10.30 maps the contact between the Dalradian Supergroup and the older Moine Supergroup in Donegal, Ireland. The vergence of minor folds is shown by the black dot/ open circle convention of Figure 10.14; larger minor folds are shown by the sinuous path of layering (foliation) which is termed an L–S fabric (L = lineation; S = schistosity). The contact between the Dalradian and Moine is a tectonic slide that is a shear zone with a normal sense of displacement. Draw a plunge profile of the folding using the average mineral lineation as the plunge of the folds. The folds in Figure 10.30 and some earlier diagrams show curvature of the fold hinges. Such folds are termed
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FIGURE 10.26 East Pembrokeshire, UK, showing folds and thrusts.
periclinal folds (or whaleback folds) and they produce variations in axial plunge and the orientation of cleavage– bedding intersection lineations. Figure 10.31(a) shows the initiation of such a fold due to heterogeneous flattening across the axial plane. A more extreme case is shown in (b) in which case the folds would be called sheath folds; younging directions are highly dispersed. An actual example of natural sheath folds is shown in (c) from Atikokan in N. Ontario. The section is approximately 10 km long, very well exposed, and abundant graded bedding and cleavage bedding relations reveal the structure as shown. Figure 10.32 is a hypothetical map of folds with axial plane cleavage. Draw a cross-section LR showing the attitude of minor structures (folds and cleavage) and then produce a plunge profile. Figure 10.33 is a famous cross-section of the SW Scottish Highlands mapped by Sir Edward Bailey (1934).
The section shows an isoclinally folded stratigraphy, but the isoclinals F1 recumbent folds have been refolded by upright F2 folds. First, trace in pencil the axial traces of the F1 and F2 folds across the section. Second, in the box below the cross-section make a sketch map of the geology in the section. Add symbols to show the orientation of bedding and first cleavage. The structural facing is the component of young projected onto the axial plane first cleavage; it yields sense of stratigraphic sequence from a single outcrop. This area of negligible topographic relief occurs near Shebandowan, N. Ontario and was mapped by W. M. Schwerdtner (1984) (Figure 10.34). It reveals a pluton that has been refolded as revealed by the orientations of deflected L and S fabrics. Draw a plunge profile of the F2 fold and mark on it the orientation of refolded S1. Are the minor folds at the hinge of the pluton’s major fold F1 or F2?
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FIGURE 10.27 Gananoque East area, Canada, showing multiply folded and metamorphosed rocks of the Canadian Shield (Map after Hewitt, 1964).
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FIGURE 10.28 Plunging anticline of Pre-Cambrian slates in Eastern England. Produce a plunge profile (see text).
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FIGURE 10.29 Overturned synform in the south (Loch Awe Syncline, SW Scotland.) Cleavage–bedding relationships and way up help reveal the structure (draw a section LR).
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FIGURE 10.30 Tightly folded high-grade schists and psammites, Donegal. Minor fold asymmetry (vergence) reveals the major structure.
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FIGURE 10.31 Examples of kilometer size folds from Atikokan, N. Ontario. The complex fold forms are due to severe plastic deformation progressing from the upper diagrams (a and b) to the final state in the lower diagram (c).
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FIGURE 10.32 Hypothetical folding. Draw a plunge profile.
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FIGURE 10.33 Cross-section of part of SW Scottish Highlands by E. Bailey (1934). The area shows two major fold episodes. Mark the axial traces of the F1 and F2 folds and complete a sketch map in the box below.
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FIGURE 10.34 Map of NW Ontario by W. M. Schwerdtner showing two major phases of folding.